Validation Test Cases Research Articles

SPH-DEM model for free-surface flows containing solids applied to river ice jams

ABSTRACTA meshless method is used to simulate free-surface fluid flows containing solid particles, motivated by the need to simulate river ice dynamics problems. A smoothed particle hydrodynamics model (SPH), with an arbitrary Lagrangian–Eulerian formulation for the fluid phase, is two-way coupled with the discrete element method (DEM) for the solid phase. Validation test cases include a bouncing sphere on a level surface, a collapse of a granular column, wedge entry into still water and solids of different densities falling into still water. The computed results using the SPH-DEM model agree quantitatively with the expected behaviour in the test cases. Numerical convergence is demonstrated for the wedge entry validation case. The SPH-DEM model is then used to simulate the stability of floating ice blocks approaching a stationary cover and ice accumulation upstream of an obstruction. The results show promise to serve as a useful quantitative engineering tool.

A condensed transfer function method as a tool for solving vibroacoustic problems

Substructuring approaches are nowadays widely used to predict numerically the vibroacoustic behavior of complex mechanical systems. Some of these methods are based on admittance or mobility frequency transfer functions at the coupling interfaces. They have already been used intensively to couple subsystems linked by point contacts and enable to solve problems at higher frequency while saving computation costs. In the case of subsystems coupled along lines, a Condensed Transfer Function method is developed in the present paper. The admittances on the coupling line are condensed in order to reduce the number of coupling forces evaluated. Three variants are presented, where the transfer functions are condensed using three different functions. After describing the principle of the CTF method, simple structures will be given as test cases for validation.

Prediction of cavitation erosion for marine applications

The paper presents the development of a cavitation erosion prediction method. The approach is tailored to marine applications and embedded into a VoF-based procedure for the simulation of turbulent flows. Supplementary to the frequently employed Euler-Euler models, Euler-Lagrange approaches are employed to simulate cavitation. The study aims to convey the merits of an Euler-Lagrange approach for erosion simulations. Accordingly, the erosion model is able to separate different damage mechanisms, e.g. micro-jets, single and collective bubble collapse, and also quantifies their contribution to the total damage. Emphasis is devoted to the prediction of the cavitation extend, the influence of compressible effects and the performance of the material damage model in practical applications. Examples included refer to 2D validation test cases and reveal a fair predictive accuracy.

A semi-empirical method for assessing the performance of an open-drive screw refrigeration compressor

A semi-empirical modeling procedure for twin-screw compressor performance simulation is proposed in this paper. In detail, the compression process is split into a number of stages, taking into account fluid leakages, heat transfers as well as heat and power losses. Compared to the simplified models reported in technical literature for other positive-displacement units, a different formulation for ambient heat loss is proposed, while mechanical power losses are modeled according to torque losses depending on both load and viscous friction effects. The proposed model applies to open-drive compressors, so three specific units for refrigeration applications with medium-high evaporation temperature are selected as the test cases for model tuning and validation, based on manufacturer's catalog data. After the identification of eight parameters, the model is able to compute the mass flow rate, the shaft power and the fluid discharge temperature, as well as volumetric and compressor efficiencies and ambient heat losses.Contrary to geometric models, the semi-empirical procedure proposed in this study does not take the presence of lubricating oil into account, as (i) oil injection with no external cooling results in a thermal neutral process; (ii) the compression work for the oil is negligible in comparison with the one required by the refrigerant fluid; and (iii) even injecting oil at a lower temperature, no considerable saving in refrigerant specific work can be achieved. Actually, this hypothesis does not significantly affect the output of the model. As a matter of fact, referring to the first test case with the larger displacement volume and to the identification of the eight model parameters based on eight convenient data points, the mass flow rate delivered by the compressor is predicted with a maximum error up to 2.44%, and the shaft power is predicted with an even limited error, except for few points with the lowest and highest pressure ratios, where the shaft power is under-estimated by 5%. Moreover, fluid temperature at the compressor exhaust is calculated with an error within ±1 K for pressure ratios up to 7.5. Similarly interesting results are achieved for the other two test case compressors, with absolute values of the errors in mass flow rate and shaft power predictions always less than 3% and 5% respectively.Finally, further considerations on the actual possibility of generalizing the model to semi-hermetic units are suggested.

An Approach for Cluster-Based Retrieval of Tests Using Cover-Coefficients

Retrieving relevant test cases is a recurring theme in software validation. We present an approach for cluster-based retrieval of test cases for software validation. The approach uses a probabilistic notion of coverage among line-based test profiles and can potentially discover groups of test cases executing a small number of unique lines. The distribution of lines across test profiles are analyzed to determine the number of clusters and generate a clustering structure without any additional user input. We also propose a novel and simple approach to identify test cases that are affected by software changes based on test profiles. It is shown that the clustering structures generated can be used to select affected tests economically to produce high quality regression test suites. The approach is applied to four unix utility programs from a popular testing benchmark. Our results show that the generated number of clusters and their average sizes closely track their estimates based on test profiles. The retrieval of affected tests using the clustering structure is economical and produces a good quality regression test suite.

An experimental and computational study of the flow pattern in a refrigerant ejector. Validation of turbulence models and real-gas effects

This investigation presents a study based on experimental and numerical flow pattern in a R134-a ejector. Furthermore, the entrainment ratio has also been analysed. The geometry of the ejector has a long tapered mixing chamber designed to improve pressure recovery. The numerical simulation is used to explain the flow features related with the static pressure profiles measurements, as well as a test case for the CFD validation. In this regard, an initial study has been undertaken in order to gain an insight into the CFD uncertainties in three relevant cases related to the ejector flow physics, namely turbulence modelling, real gas thermodynamics and shock-wave turbulent boundary layer interaction.The double-choked and mixed regime have been realized in this study. Both the standard k−ε and the SST k−ω turbulence models show good agreement between the experimental and computed pressure profiles. Regarding the entrainment ratio prediction, it was better for the latter in the double-choked regime. However, for the mixed regime, it was found that the numerical analysis could not yield a reliable entrainment ratio value using any of the turbulence models employed, although the pressure profile was accurately computed. An explanation for this contradiction has been found in this investigation. Furthermore, the numerical analysis has helped to explain the differences found between the measured pressure profiles and the one-dimensional theory prediction.

Numerical Method for Calculation of Complete Casting Processes—Part II: Validation and Application

In this article, the numerical method proposed in Part I is validated by applying it to relatively simple validation test cases with phase change as well as to real-life problems. First, the results of the 1-D calculations are compared with the analytical solutions for the Stefan problem with mushy zone. Then, the 2-D and 3-D calculations of the Bridgman crystal growth are compared with available experimental results. The ability to predict the residual stresses is demonstrated on an academic 2-D example. Finally, the results of calculation for two real-life industrial cases, injection casting and solidification of a multicomponent metal drill head, are presented and discussed.

Calculation of fluid–structure interaction: methods, refinements, applications

This paper starts by reviewing the most significant previous studies on fluid–structure interaction of flows past stationary and moving circular cylinders, including those undergoing fluid dynamic excitation in the form of vortex-induced vibrations. It shows the importance of flow past a circular cylinder as a convenient validation test case for new numerical methods and demonstrates the advantages of the immersed boundary method. A new method based on the immersed boundary method to accurately compute fluid–structure interaction and vortex-induced vibrations is then presented and applied. The fluid solver is strongly coupled with the equation of motion through Hamming's fourth-order predictor–corrector method, which allows computation of the interaction of a fluid with a moving structure. The accuracy, validity and efficiency of the proposed method are demonstrated by a series of validation cases, including flow past stationary and moving cylinders, both prescribed and freely vibrating. The numerical method reproduces fairly well the vibration of an elastically mounted cylinder subjected to a fluid flow. In particular, vibration amplitude as well as vibration frequency are computed quite accurately. In addition, the predicted vortex-induced cylinder vibration lock-in regime is in good agreement with laboratory experiments.

Benchmarking nuclear fission theory

We suggest a small set of fission observables to be used as test cases for validation of theoretical calculations. The purpose is to provide common data to facilitate the comparison of different fission theories and models. The proposed observables are chosen from fission barriers, spontaneous fission lifetimes, fission yield characteristics, and fission isomer excitation energies.

Use of Genetic Approach for Test Case Prioritization from UML Activity Diagram

testing plays an important role for evaluation of the quality of the software. Quality of the software depends upon the kind of testing strategies applied by the software tester who generates valid and invalid test cases for evaluating the quality of software. For optimizing the testing procedure, testing paths are very valuable to judge the quality of the software. From the literature, it is revealed that it is big challenge to optimize the testing paths. In the present work, the concept of genetic algorithm is used for prioritization of test cases generated from Unified Modeling Language. Testing paths are generated from activity diagram is designed for generation of test cases from design specification which will further reduce the cost of software testing. Branch coverage technique and predicate coverage methods are used for prioritization of test cases by identifying the independent paths. Information flow metric and decision node based genetic function are used through a case study.

Boundary element and finite element coupling for aeroacoustics simulations

We consider the scattering of acoustic perturbations in the presence of a flow. We suppose that the space can be split into a zone where the flow is uniform and a zone where the flow is potential. In the first zone, we apply a Prandtl–Glauert transformation to recover the Helmholtz equation. The well-known setting of boundary element method for the Helmholtz equation is available. In the second zone, the flow quantities are space dependent, we have to consider a local resolution, namely the finite element method. Herein, we carry out the coupling of these two methods and present various applications and validation test cases. The source term is given through the decomposition of an incident acoustic field on a section of the computational domain's boundary. Validations against analytic, another numerical method and measurements on different test cases are presented.

Benchmarking in a rotating annulus: a comparative experimental and numerical study of baroclinic wave dynamics

The differentially heated rotating annulus is a widely studied tabletop-size laboratory model of the general mid-latitude atmospheric circulation. The two most relevant factors of cyclogenesis, namely rotation and meridional temperature gradient are quite well captured in this simple arrangement. The radial temperature difference in the cylindrical tank and its rotation rate can be set so that the isothermal surfaces in the bulk tilt, leading to the formation of baroclinic waves. The signatures of these waves at the free water surface have been analyzed via infrared thermography in a wide range of rotation rates (keeping the radial temperature difference constant) and under different initial conditions. In parallel to the laboratory experiments, five groups of the MetStrom collaboration have conducted numerical simulations in the same parameter regime using different approaches and solvers, and applying different initial conditions and perturbations. The experimentally and numerically obtained baroclinic wave patterns have been evaluated and compared in terms of their dominant wave modes, spatio-temporal variance properties and drift rates. Thus certain “benchmarks” have been created that can later be used as test cases for atmospheric numerical model validation.

Numerical Model to Analyze Transient Behavior and Instabilities on Hybrid Rocket Motors

The study of hybrid rocket transient behavior is a very important issue in order to analyze instabilities and develop throttleable motors. In this paper, an unsteady numerical model is presented. The model is composed of three submodels linked together: 1) zero-dimensional model of the combustion chamber, 2) one-dimensional (1-D) radial model of the fuel grain, and 3) zero-dimensional model of the feeding system. The first model simulates combustion chamber dynamics. The one-dimensional radial model of the fuel grain includes both standard polymeric and liquefying propellants characterized by a melting layer. The fuel block models the heat exchange to the wall, the heat propagation through the solid/liquid phase, and the fuel surface evaporation/entrainment. Finally, the injection system block simulates the unsteady behavior of the feeding line and droplets break-up-evaporation dynamics. In this paper, all the blocks are presented together with their validation versus analytical test cases. For each block, an analysis of the effect of the influencing parameters on the transient thruster behavior is presented.

Modeling for Minimization of Loneliness of People on the Social Networking Websites

In the current scenario of human life, use of Internet is an integral part of human being. Internet users are rapidly increasing worldwide. Use of internet is directly or indirectly affecting the life of human being as there are several advantages and disadvantages for using the services of internet especially for the online shopping, reading, searching articles, watching movies, net surfing, etc. In the present work, a real case study of domain of Indian people is considered and questionnaire for Internet Addiction Test is prepared and statistical analysis is used to find out some of important observations related to human being which are using the internet services. Although in India, rate of internet users is very poor however Internet Addiction Test shows some significant results which minimize the loneliness of human being due to vast use of social networking sites. A model is also proposed by the use of most popular object-oriented modeling platform independent language known as the Unified Modeling Language. One can easily develop the software code through proposed model by the use of any object-oriented programming language. Systematic steps are represented through model for minimization of loneliness of people which is converted into happiness by the use of the social networking websites. The proposed model is also validated by generation of valid test cases through Finite State Machine.

Underexpanded Jet Development from a Rectangular Nozzle with Aft-Deck

An experimental study is reported of underexpanded supersonic jet plumes issuing from a high-aspect-ratio convergent rectangular nozzle. Schlieren visualization, Pitot probe, and Laser Doppler Anemometry measurements are carried out to capture the plume development in the near field, and in particular the effect on the plume flow of a finite-length extended shelf or aft-deck attached to the lower nozzle wall. This creates asymmetry in the inviscid shock cell pattern and the entrainment characteristics, both of which influence shear-layer growth and plume trajectory. A net pressure force is induced on the aft-deck wall, which leads to transverse deflection of the jet plume once it leaves the aft-deck, both upward and downward, depending on aft-deck length and nozzle pressure ratio. For sufficiently high nozzle pressure ratio and a sufficiently long aft-deck, separation and reattachment of the plume from the aft-deck is observed. Detailed mapping of both mean velocity and turbulence in the plume near field has been carried out, enabling comparison of flow behavior for a clean nozzle and a nozzle with aft-deck. The data provided are proposed as a suitable benchmark validation test case for computational fluid dynamics studies of rectangular nozzle plumes with aft-deck interaction effects.

Sensitivity analysis to unsteady perturbations of complex flows: a discrete approach

SUMMARYA discrete framework for computing the global stability and sensitivity analysis to external perturbations for any set of partial differential equations is presented. In particular, a complex‐step approximation is used to achieve near analytical accuracy for the evaluation of the Jacobian matrix. Sensitivity maps for the sensitivity to base flow modifications and to a steady force are computed to identify regions of the flow field where an input could have a stabilising effect. Four test cases are presented: (1) an analytical test case to prove the theory of the discrete framework, (2) a lid‐driven cavity at low Reynolds case to show the improved accuracy in the calculation of the eigenvalues when using the complex‐step approximation, (3) the 2D flow past a circular cylinder at just below the critical Reynolds number used to validate the methodology, and finally, (4) the flow past an open cavity presented to give an example of the discrete method applied to a convectively unstable case. The latter three (2–4) of the aforementioned cases were solved with the 2D compressible Navier–Stokes equations using a Discontinuous Galerkin Spectral Element Method. Good agreement was obtained for the validation test case, (3), with appropriate results in the literature. Furthermore, it is shown that for the calculation of the direct and adjoint eigenmodes and their sensitivity maps to external perturbations, the use of complex variables is paramount for obtaining an accurate prediction. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of molecular transport on PDF modeling of turbulent non-premixed flames

Comprehensive studies are performed to evaluate the effect of molecular transport on the probability density function (PDF) modeling of turbulent non-premixed flames. The widely used Sandia piloted jet flame E is chosen as the validation test case. Three configuration cases are considered: neglecting the molecular transport (Case-A), considering the molecular transport (Case-B), and considering both the molecular transport and its effect on mixing (Case-C). Extensive testing is done to assess the effect of molecular transport on the performance of the different mixing models and on the prediction of the flame extinction limit. It is found that the molecular transport has a significant effect on the prediction of the upstream mixing layer between the pilot stream and the coflow, where the turbulence level is weak or it is laminar and hence the molecular transport dominates the turbulent transport. Accounting for the molecular transport increases the models’ resistance (for the IEM model and the modified Curl model) to flame extinction and yields a burning flame with a lower mixing rate that can cause global extinction without molecular transport. A modification to the mixing frequency is introduced to account for the effect of the molecular diffusion on mixing. With such a modification, the modeled flame sustainability is further enhanced and a lower mixing rate can be used, which enhances the ability of the IEM model and the modified Curl model which were previously found to yield a burning flame solution only at a highly increased rate of mixing.

Zonal Immersed Boundary Conditions: Application to a High-Reynolds-Number Afterbody Flow

The present paper focuses on a new numerical strategy consisting of the zonal use of the immersed boundary conditions combined with the ability of zonal detached-eddy simulation to simulate high-Reynolds-number separated flows. The validation test case is a simplified space launcher afterbody with a control device in the form of a short cylindrical serrated skirt. “Zonal immersed boundary conditions” are used to handle the introduction of the skirt into the preexisting structured curvilinear grid that describes the noncontrolled configuration. The governing equations are solved using a standard body-fitted finite volume technique over the whole grid. A direct forcing source term is added when cells are internal to the skirt (that is, solid) to drive the velocity and the turbulence variables to the chosen values. Numerical simulations are performed at a Reynolds number of and a freestream Mach number of 0.702. The numerical results demonstrate the ability of the zonal immersed boundary conditions to successfully impose the desired values at solid nodes. The first- and second-order moments, as well as the fluctuating pressure field, illustrate a fairly good agreement between the experiment and the numerical simulation. It is shown that the serrated skirt acts as a smooth device, which does not affect the global dynamics of the flow and only delays the location of the separation. Finally, the zonal immersed boundary conditions appear to successfully reproduce the effect of the skirt while drastically reducing the time of the grid generation.

Prediction of fracture in a dual-phase steel subjected to non-linear straining

In this work, selected fracture criteria are applied to predict the fracture of dual-phase steel subjected to non-linear strain paths. Furthermore, the effects of manufacturing history are studied. Four fracture criteria were calibrated in three tests using standard specimens. The fracture criteria were first validated in the circular Nakajima test. A second validation test case was included in order to validate fracture prediction for non-linear strain paths. In this test a sheet metal component was manufactured and subsequently stretched until it fractured. All fracture criteria included in this study predict fracture during the Nakajima test with reasonable accuracy. In the second validation test however, the different fracture criteria show considerable diversity in accumulated damage during manufacturing which caused substantial scatter of the fracture prediction in the subsequent stretching. This shows that manufacturing history influences the prediction of fracture.

Determination of the Lumped-Capacitance Parameters of Air-Cooled Servers Through Air Temperature Measurements

Computer servers can be represented by lumped thermal capacitances for the purpose of simulating server and data center transient thermal response to changes in operating conditions or equipment failures. Two parameters are needed to characterize the transient behavior of a lumped-capacitance server: its thermal capacitance and its thermal conductance, heat transfer effectiveness, or time constant. To avoid the laborious task of obtaining these parameters from measurements or estimations of the thermal characteristics of internal components of the server, a method is proposed to derive these parameters from external measurements that can be easily obtained without performing an “autopsy” on the server. In this paper, we present the mathematical formulation underlying the proposed method and describe how the parameters are to be obtained from external air-temperature measurements using the mathematical model. We then present validation test cases using experimental data from server shut-down and inlet-temperature ramp tests. The experimentally obtained parameters are implemented into a computational fluid dynamics (CFD) case study of server shutdown in which the transient server exit air temperature is computed from the lumped-capacitance parameters via a user-defined function. The results thus obtained are in excellent agreement with the experimental data.